1. Field of the Invention
The present invention relates to an image pickup apparatus which can obtain an accurate clamping operation.
2. Description of the Prior Art
Generally, to prevent blooming in solid state image sensors such as a CCDs and the like, there have been considered a method whereby an overflow drain is provided in a photosensing surface and a method whereby the overflow carrier is extinguished using surface recombination, thereby controlling the storage state in the photosensing surface.
The latter method, in particular, since an aperture efficiency in the photosensing surface is not sacrificed, has advantages that sensitivity is high and the degree of integration can be improved, thereby enabling the horizontal resolution power to be increased, and the like.
FIGS. 1 to 3 show diagrams to describe a principle of such a blooming preventing method by way of the surface recombination, in which FIG. 1 shows a front view of a typical frame transfer type CCD.
In the diagram, a reference numeral 1 denotes a photosensing part consisting of a plurality of vertical transfer registers having photosensitivity.
Also, a reference numeral 2 indicates a storage part consisting of a plurality of light shielded vertical transfer registers.
A numeral 3 is a horizontal transfer register and it is possible to fetch the information in each vertical transfer register of the storage part 2 into this horizontal transfer register by simultaneously shifting such information by one bit, and then by allowing the register 3 to perform the horizontal transfer operation, a video signal can be derived from an output amplifier 4. OB represents a vertical transfer register part which is light shielded in the vertical direction.
Generally, the information formed in each vertical transfer register of the photosensing part 1 is vertically transferred into the storage part 2 during the vertical blanking interval in the standard television system and is sequentially read out line by line from the horizontal transfer register 3 during the next vertical scanning interval.
It is now assumed that the photosensing part 1, storage part 2 and horizontal transfer register 3 are respectively two-phase driven; that their respective transfer electrodes are P.sub.1, P.sub.2, P.sub.3, P.sub.4, P.sub.5, and P.sub.6 ; and that their transfer clocks are .phi..sub.P1, .phi..sub.P2, .phi..sub.P3, .phi..sub.P4, .phi..sub.P5, and .phi..sub.P6.
FIG. 2 is a diagram showing a potential profile under such transfer electrodes P.sub.1 to P.sub.6. For example, a low potential section and a high potential section are formed by way of ion implantation or the like under each electrode provided over a p-type silicon substrate 6 through an insulating layer 5. For instance, when a low level voltage -V.sub.1 is applied to the electrodes P.sub.2, P.sub.4 and P.sub.6 and a high level voltage V.sub.2 is applied to the electrodes P.sub.1, P.sub.3 and P.sub.5, a potential as indicated by the solid line in FIG. 2 is formed. On the other hand, when a low level voltage -V.sub.1 is applied to the electrodes P.sub.1, P.sub.3 and P.sub.5 and a high level voltage V.sub.2 is applied to the electrodes P.sub.2, P.sub.4 and P.sub.6, a potential as indicated by the broken line in the diagram is formed.
Therefore, by applying alternating voltages with mutually opposite phase to the electrodes P.sub.1, P.sub.3, P.sub.5 and to the electrodes P.sub.2, P.sub.4, P.sub.6, the carriers are sequentially transferred in the negative (-) direction (to the right in the diagram).
In addition, the alternate long and short dash line in the diagram indicates the potential in case of applying a large positive voltage V.sub.3 to the electrodes. Since the well of this potential becomes the inversion state, the overflow carrier over a predetermined quantity will have been recombined with the majority carrier and will have disappeared.
FIG. 3 is a diagram showing the relation between such an electrode voltage and a shape of the internal potential in the direction of thickness of the semiconductor substrate 6. As can be seen from this graph, the potential well for the electrode voltage V.sub.3 becomes shallow and becomes a second state in that the overflow carrier recombines with the majority carrier at the interface with the insulating layer.
On the other hand, the potential becomes an accumulation state as a first state at the electrode voltage -V.sub.1. In this state, the majority carrier can be easily collected around the interface and this majority carrier is supplied from, for example, a channel stopper region (not shown).
Therefore, by alternately applying voltages -V.sub.1 and V.sub.3 to the electrode P.sub.1 in the state in that a barrier is formed by applying, for example, the voltage -V.sub.1 to the electrode P.sub.2, the minority carrier to be accumulated under the electrode P.sub.1 is limited to a quantity below a predetermined quantity.
However, on the contrary, to effectively extinguish the overflow carrier, the accumulation state and the inversion state have to be alternately formed at a high speed in the semiconductor substrate during the accumulating interval. Therefore, there is a problem that electric power consumption is large. There is also a problem that when such pulse control is performed at high speed, a noise to be caused due to this pulse is mixed into the signal.
FIGS. 4A and 4B are diagrams for describing such problems. In the diagrams, a reference numeral 100 denotes a part of a driver circuit which will be described later and it also serves to supply a driving pulse (hereinbelow, referred to as an AB pulse .phi..sub.AB) at predetermined peak to peak levels -V.sub.1 and V.sub.3 in response to timing of a pulse .phi..sub.ab from a clock generator which will be described later.
Reference numerals 101 and 109 denote differentiating capacitors; 102 and 108 are biasing diodes; 104 and 107 are transistors; 103 and 106 are smoothing capacitors; and 105 is an interelectrode capacitance of the electrode P.sub.1.
FIG. 4B is a diagram showing the waveforms at each part. The operation of the circuitry shown in FIG. 4A will now be described in conjunction with FIG. 4B.
As shown in FIG. 4B, when the pulse .phi..sub.ab is input, the transistor 107 is turned on in response to its leading edge, so that a current i.sub.AB flows from the capacitance 105 toward the power supply -V.sub.1 and this power supply -V.sub.1 is applied to the capacitance 105 and is charged therein.
In addition, the transistor 104 is turned on in response to the leading edge of the pulse .phi..sub.ab, so that the power supply +V.sub.3 is applied to the capacitance C and is charged at this voltage V.sub.3. In this case, since the interelectrode capacitance 105 has equivalently an input capacitance of some 1000 pF, when the pulse at the voltages -V.sub.1 and V.sub.3 (hereinbelow, abbreviated as AB pulse .phi..sub.AB) is applied, a differential current of a few A (amperes) will have flowed instantaneously. When this current flows through the silicon substrate of the image sensor, a noise of a few 10 mV (which is called an AB noise here) will have been eventually produced since a resistance of the silicon substrate has a value of a few 10 m.OMEGA.. To reduce this noise, methods can be considered whereby the resistance of the silicon substrate is decreased and whereby an absolute value of a differential current is diminished by making the leading and trailing characteristics of the AB pulse smooth; however, the AB noise of a few mV will have remained even by these methods.
On the other hand, a standard level of the signal to be output from the image sensor is ordinarily a few 100 mV and when the dynamic range at the image pickup time is set to be about four times the standard signal level, a general video signal level will be a value of the order of about a few 100 mV as well.
Therefore, in a general movie image pickup device, in the case where an object is particularly dark, the AB noise having a level of the order which cannot be ignored appears on a screen.
In addition, as for an output signal of the image sensor, the signal corresponding to the optically shielded section OB is generally clamped as a black reference signal by a DC reproducing circuit called a clamping circuit. When a pulse is supplied in the interval of this black reference signal, the AB noise will have been added to the black reference signal. Therefore, when this signal is clamped, a variation in clamping potential due to the AB noise is caused and this becomes a line-drawing like low frequency noise on display, causing a picture quality to remarkably deteriorate.
Although a variation rate of this clamping potential is not determined by only a ratio between the foregoing standard signal level (100 mV) and the level (a few mV) of the AB noise, it has been confirmed that the noise of a few 10 mV remains with regard to the NTSC signal level even when the clamping effect, gamma characteristics, and the like are considered. In addition, such a phenomenon is remarkable particularly in the case where the AB pulse .phi..sub.AB is asynchronous with the TV synchronization (for example, horizontal synchronization), where the repetitive period of the AB pulse is changed in accordance with the luminous level or the like of the object, where the phases of the clamp pulse and AB noise were changed, or the like.
Furthermore, as another method to control the storage state at the photosensing surface of the image pickup device, a method is considered whereby the accumulated charges are once cleared in one field to control the substantial storage time while performing the accumulation in the photosensing surface at the field period of the standard television system.
A problem in such a method for controlling the storage state will be described hereinbelow.
As for an ordinary video camera, in case of performing the image pickup synchronously with the vertical sync signal, an exposure time, i.e., a shutter speed is fixed at 1/60 second since the repetitive frequency of the vertical sync signal is 60 Hz. However, in case of photographing an object which is moving at a high speed, to obtain a sharp picture image without blur, it is necessary to make the shutter speed variable, in particular, to make it variable in order to have high speed. Particularly, this is important in a still video system which performs one shot photography.
In case of obtaining a shutter speed shorter than 1/60 second, there is a method whereby, for instance, a rotary shutter or the like is used synchronously with the vertical sync signal, or the like; However, this method has a drawback such that when the exposure time becomes short, it is difficult to follow mechanically or its mechanism becomes complicated, and the like. On the other hand, there is a method as shown in FIG. 5 for performing the signal processing. That is, for the ordinary driving period of accumulation and transfer, this method is performed in the manner such that by supplying a vertical transfer clock as indicated by b in FIG. 5 to the transfer electrode of the photosensing part at appropriate timing, the charges accumulated in the image sensor during the video interval are once thrown through the overflow drain in the photosensing surface and the new accumulation is begun from time t.sub.1. According to this method, since any special mechanism is unnecessary, the apparatus can be miniaturized. However, to generate the pulse as indicated by b during the video interval, a leakage pulse remains in the signal read out by the image sensor similarly as in FIG. 4B, so that this causes a bad influence on a picture image. Referring now to FIG. 6, there is shown the relation between the clamp pulse and the video signal. Although, ordinarily, a part of the image sensor is light shielded and an output of this portion is clamped as a reference black (or optical black) level, when the foregoing clear pulse b is generated during the video interval, its leakage component appears as a form (a' in FIG. 7) of which it was added to an output signal component a. Thus, when this leakage component has been once superimposed in this clamping portion, the black level will have been varied, so that there is a drawback such that the luminous level will have been varied for several H.